The invention relates to an electric incandescent lamp comprising:
a light-transmitting lamp vessel which is closed in a vacuumtight manner and which has a longitudinal axis; PA1 a helically coiled tungsten incandescent body inside the lamp vessel, substantially concentric with the axis; PA1 current conductors each having a straight inner conductor which is substantially concentric with the axis and which has an end portion surrounded by turns integral with the incandescent body, which current conductors issue from the lamp vessel to the exterior.
Such an electric incandescent lamp is known from EP-A-0 358 061.
It is important for some electric incandescent lamps that the incandescent body should be substantially concentric with the longitudinal axis, for example when the lamp vessel has an IR-reflecting filter, or when a centered position of the lamp in an appliance is to achieve a centered position of the incandescent body of the lamp in the applicance.
The incandescent body of the known lamp has a number of turns at both sides and integral with the body of a diameter which is relatively small compared with that of the light-emitting turns of the incandescent body and which merge into end turns of comparatively large diameter. The latter turns are in circumferential contact with the lamp vessel and are held centered thereby.
The inner conductor extends through these end turns, surrounded thereby at a considerable distance, and is threaded into the turns of comparatively small diameter.
The known lamp is of a complicated construction which is difficult to reproduce. The incandescent body with the integral portions thereof on either side comprises turns of three different diameters. These complicate the manufacture of the incandescent body considerably and make its cost high. The incandescent body is supposed to be centered in the lamp vessel by the turns of greatest diameter. These turns, however, are connected to the incandescent body only by means of the flexible wire from which the incandescent body was coiled, so that centering of the former turns does not provide a guarantee for centering of the incandescent body itself.
The inner conductor is merely threaded into the turns of smallest diameter, so that a bad electrical contact is to be feared. Furthermore, manipulation of the incandescent body during threading may eliminate the centering effect.
Other constructions described in the cited document also suffer the disadvantage of complicated shapes such as, for example, an incandescent body having turns which widen continuously from the center to the ends and in which conically coiled inner conductors are accommodated whose free ends are in addition bent away along the centerline of the relevant incandescent body.